Open Source Unix like Operating system and its based on Linux Kernel. Popular Linux distributions include Debian , Fedora and Ubuntu ( I like the most). Commercial distributions include Red Hat Enterprise and SUSE Linux.

Linux OS is widely used and used even in TV, Cars , Routers, web applications , Video games consoles.

UnixMulti-tasking, single- or multi-user OS. Predominant server OS on Internet servers; has always used TCP/IP.
LinuxOpen-source, lightweight version of UNIX; many variants. Excellent OS for resource sharing, uses TCP/IP

LINUX is not at all synonym of UNIX but it was clone of UNIX and adds other variety of features. Unix was commercial and was basically used in companies like IBM, HP and Sun Solaris.

Lets get Started on How Linux or Ubuntu BOOTS up :

CPU Starts —–> CPU Gets first Instruction from Memory i.e. ROM BIOS—> ROM BIOS reads MBR and runs  —> POST checks such as HARDWARE’S  —> Finally BIOS looks in CMOS chip after reading MBR and Boot loader for OS.

Important Commands in Linux :

System uptime informationuptime
kernel informationuname -r 
IP address of machinehostname -I
last rebootlast reboot
Who you are logged in aswhoami
who are onlinew or who
CPU informationcat /proc/cpuinfo
Memory Informationcat /proc/meminfo
free and used memory free -h
hardware infodmidecode
List files opened by userlsof -u user
all the currently running processesps -ef
your currently running processesps
mysql processps –ef | grep mysql
Date & Time Settingschronyd
Date & Time Settingsntpd
Displaying the Current Date and Timedate
Displaying the Current Date and Timetimedatectl
Command to manage systemdsystemctl
System-wide locale settings/etc/locale.conf
Listing available system locale settings:localectl list-locales
current status of the system locales settingslocalectl status
configuring network accessnmcli
configuring network accessnmtui
modify the existing connection:nmcli con mod “con-name”
display all connections:nmcli con show
display the active connection:nmcli con show –active
Linux Softwares areRPM packages
Repo Directory/etc/yum.repos.d/
available repositories:subscription-manager repos –list
currently enabled repositories:yum repolist
Searching for packagesyum search string
Installing a package:yum install package_name
Updating all packagesyum update
Updating a package:yum update package_name
Uninstalling a packageyum remove package_name
installed and available packagesyum list all
installed packages:yum list installed
enable a servicesystemctl enable service_name
disable a servicesystemctl disable service_name
Firewall service firewalld
current status of firewalldsystemctl status firewalld
Start firewalldsystemctl start firewalld
additional layer of system securitySELinux
Display the current SELinuxgetenforce
Change the state of SELinuxsetenforce Enforcing/Permissive
Permanent SELinux State/etc/selinux/config
SSH connectionfacilitates client-server communication
Disabling SSH Root Loginvi /etc/ssh/sshd_config
PermitRootLogin no
systemctl restart sshd
Min value in useraddUID_MIN 1000
Max value in useraddUID_MAX 60000
Adding a user to a group:usermod -a -G group_name user_name
<1000Reserved userID details in/etc/login.defs